ABSTRACT: We investigated the possible biotic and abiotic causes of the observed long-term decrease of oxygen minimum concentrations in the deep water of Gullmar Fjord. Physical factors explained about 40% of the decreased minimum oxygen concentrations since the 1950s. The North Atlantic Oscillation and more regional climate variations account for an important part of this change. The effect of the climate variations was mainly on the timing of the renewal of the basin water. The consumption rate of oxygen in the basin water has increased by 50% since the 1950s. This biotic effect explained about 60% of the decrease in minimum oxygen concentrations. The likely dominating source of oxygen-consuming matter was (remote) production in the Skagerrak, suggesting that there has been a long-term increase of particulate organic carbon in the Skagerrak water.

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